A stranger was seen by someone to walk into the building .
加油!不明白再问!句型结构是什么？固定用法：see sb do sth变被动结构时TO 要还原，即： sb be seen to do sth 根据SAW 可知是一般过去时，故BE 要用WAS(主语a stranger 是三单） ,SEE的过去分词是 seen